1. Field of the Invention
The present invention relates to the field of refrigerant, compressed gas or air dryer systems, and more particularly to a variable frequency drive process to control compressor speed to yield higher quality gas or air dew point output at significant energy savings. The invention also relates to algorithms and methods used to reduce or eliminate undesired or unnecessary dryer cycling.
2. Background
Presently, many industrial applications using air or gas driven machinery have a need for dry air or gas in the process of operating, product process, product fabrication, as well as many other applications. Air or gas driven machinery is most commonly operated using pressurized, i.e., compressed air or gas that contains water that can react on or condense within product or apparatus and negatively impact the air or gas usefulness. Moisture in the form of condensation or precipitation in machinery or on product negatively impacts the product process systems by causing costly equipment maintenance or equipment failure and befouled product.
Refrigerant dryers are the most common devices to remove moisture from compressed air or gas for such industrial uses, thus reducing failures and improving product quality. The water content quality of the air or gas being dried, at the dryer's output is measured in terms of dew point, the temperature where water vapor in the air or gas is at 100% humidity; the lower the dew point temperature, the greater the dryness of the air or gas. Dryer air or gas is considered higher quality. Industry desires a gas or air of sufficient quality to prevent water from damaging machinery or fouling product.
There are several types of refrigerant air or gas dryers, the following list includes more conventional systems: Cycling Dryers, Non-Cycling Dryers and Variable Speed Drive Dryers. In general, refrigerant air or gas dryers have: 1) a refrigerant compressor (with an appropriate accumulator and receiver); 2) a series of heat exchanger vessels and/or other ‘heat’ transfer components; 3) a condensing component; and a 4) a refrigerant process controller having one or more of the following: expansion, pressure regulating, bypass valves; solenoids and electronic sensors/controls; an optional variable speed drive (VSD) system for the compressor motor.
These systems all operate on various levels of efficiency, both with respect to cost and dew point performance. In common practice, a certain type of cycling air or gas dryer includes an unloading feature that to allows the compressor motor to power down, i.e., to coast or free wheel during periods of low demand for refrigerant cooling. Thus, a cycling dryer is considered an energy savings dryer when compared to a conventional non-cycling system.
Another example of energy savings may be found in a system configured with a variable speed drive (VSD) device to decrease power to or to slow-down the compressor during lull intervals, periods of less demand for refrigerant cooling. Such a system is also considered to be an energy savings dryer because the compressor consumes less energy during the lull intervals.
Although over the years many of these systems have achieved improved operating costs, with various configurations of the basic system elements as outlined above, even these improved conventional systems suffer in dew point output performance of air or gas being dried. Under these economical conditions often the dryer will function at the peril of the wellbeing of the dryer itself. Further, these conventional systems are susceptible to adverse effects due to abnormal operational stresses associated with external conditions not within the control of the dryer apparatus, e.g., high inlet temperature, high ambient over loading the dryer capacity flows, unexpected condensing high ambient conditions, etc.
As with most machinery, air or gas dryers are designed to work under defined load and environmental specifications. Design specifications are also provided relating to output volume and output quality. For example when design specifications are exceeded from unexpected demand or a changed operating environment that causes an increased load, conventional systems react by increasing demand on the refrigerant compressor. When the increased output of the refrigerant compressor does not correct operational parameters to normal ranges, continued signals are sent to the refrigerant compressor to further increase output. Eventually a set parameter such as a refrigerant discharge pressure parameter incorporated as a failsafe to protect the machinery or for personnel safety will be exceeded causing sudden refrigerant system shutdown. Sudden refrigerant system shutdown results in reduced quality in the compressed air or gas because dew point is no longer under any control. These shutdowns not only impact production schedules and plant maintenance, but also product quality suffers and waste product disposal costs are severely increased.
Another example of when design specifications are exceeded is when the source of inlet air has a temperature above the design specifications; such as 100° F. inlet raises to 110° F., 120° F. or even 130° F. or higher due to an air compressor or after cooler problem and the like. Still another example is when ambient air around the condensing unit becomes too hot; for example, in summertime conditions, in the compressor rooms or where the apparatus may have insufficient air circulation, due to fouled condenser elements. Yet another example can be found typically on water cooled system, when cooling water flow dwindles or is not sufficiently cool.
These are but a few examples of problems that can singularly, adversely affect dryer operations resulting in poor dew point performance, waste of energy and wear-and-tear on equipment. In combination, these problems most certainly could be expected to lead to damage of not only the dryer apparatus, but significantly affect down-time of compressed air or gas equipment and of the manufacturing process in which the compressed air or gas is being used. Thus although a highly efficient refrigerant air and gas dryer is desired, if the system causes undesired loss of performance and plant shut downs, the efficient system may not be cost effective.
Thus it is readily apparent that there is a longfelt need for a structure and process to effect a more efficient driving of a variable speed refrigerant compressor, specifically a system that continues to function with only a minimal negative effect on dew point when it is operating outside design parameters and will only shut down when a true protective condition is warranted. It is clear there is a need for a system that minimizes or prevents refrigerant air or gas system shut downs that result in large amounts of high dew point compressed air flowing through the dryer and into the manufacturing plant and process. The present invention seeks to alleviate the problems associated with the present art by providing a refrigerant air or gas dryer with a means to avoid sudden system shut down by continuing to function when design parameters are breached.